Bond Strength of Porcelain to Milled Sintered and Casting Base Metal Alloys

Statement of the Problem: The success of metal-ceramic restorations depends on the bond strength between porcelain and alloy. These restorations can be fabricated through different casting and computer-aided design/computer-aided manufacturing (CAD/CAM) techniques. Purpose: This study aimed to compare the bond strength of porcelain to milled sintered (Sintron) and casting (Co-Cr and Ni-Cr) base metal alloys. Materials and Method: In this in vitro experimental study, 63 rectangular bars (25×3×0.5 mm) were fabricated of three base metal alloys: casting Ni-Cr, casting Co-Cr, and milled sintered Co-Cr alloy. Feldspathic porcelain (3×8 mm) was applied at the center of each bar with 1.5 mm thickness. The specimens were thermally aged. Bond strength was evaluated through three-point flexural test. Failure mode was evaluated by optical and electron microscope. Data were analyzed with one-way ANOVA and Tukey's post hoc test (α=0.05). Results: The mean flexural bond strength of porcelain to milled sintered Co-Cr alloy (24.58±5.16 MPa) was significantly higher than that of casting Ni-Cr (21.13±6.34 MPa) (p= 0.03) and casting Co-Cr (20.98±4.84 MPa) alloys (p= 0.04). However, the two casting alloys were not significantly different in this regard (p= 0.93). The failure mode in all specimens was of cohesive type. Conclusion: Bond strength of CAD/CAM milled sintered Co-Cr alloy was better than that of the conventional casting alloys and could serve as a suitable alternative to those alloys.


Introduction
Despite the technological advances of all-ceramic restorations, metal-ceramic restorations still play an important role in dentistry [1]. Strong bond of the metalceramic interface, which is the most susceptible area for cracking, is the major prerequisite for durability of metal-ceramic restorations [2]. Development of various computer-aided design/computer-aided manufacturing (CAD/CAM) systems has increased the quality of fullcrown restorations, allowed fabrication of wax patterns by using castable materials, and eliminated numerous limitations of the conventional waxing technique [3].
Base metal alloys can be processed with CAD/CAM through two different approaches. These include addi-tive method, which includes laser sintering, and subtractive method, in which materials of maximum strength are machine-milled. However, subtractive method is costly and only few CAD/CAM laboratory systems are able to process hard presintered cobalt-chromium (Co-Cr) blocks. The newly-developed soft non-presintered Co-Cr alloy (Sintron) can be processed in milling machines at reduced cost and time, and processing steps of quite comparable to non-presintered zirconia.
Soft Co-Cr blank is processed in a material pre-state by dry milling. The material contains adhesive agents like organic binders and is milled in a green state. Then, to achieve full density, the milled structure is sintered at high temperature in an argon gas atmosphere, which slightly decreases the material volume [4]. The final density is affected by the sintering temperature and time [5]. Non-presintered alloys and their bond to porcelain have been limitedly investigated. Therefore, this study was designed to compare the bond strength of porcelain to non-presintered alloys (CAD/CAM milled sintered Co-Cr) and conventional casting alloys (casting nickelchromium [Ni-Cr] and Co-Cr). The study also aimed to evaluate the failure mode (within the porcelain or within the metal-porcelain interface), to help future studies address porcelain failure problems. The null hypothesis was that no difference exists between the nonpresintered and casting alloys neither in their bond strength to ceramic nor in their failure pattern.

Materials and Method
In this experimental in vitro study, three groups of specimens (n=21 per group) were fabricated from non-

Results
The Kolmogorov-Smirnov test revealed that porcelain    Table 1).
One-way ANOVA also showed the three alloys to be significantly different in terms of the mean percentage of cohesive failure in the opaque layer (p= 0.03). Ac-cording to Tukey's post hoc test, the mean percentage of cohesive failure in the opaque layer in Ni-Cr group was significantly higher than that in Co-Cr (p= 0.003) and Sintron alloy (p= 0.04). However, Co-Cr and Sintron base metal alloys were not significantly different in this regard (p= 0.32). The mean percentage of cohesive failure in the body layer was significantly different among the three alloys (p= 0.009); being significantly lower in Ni-Cr alloy than in Co-Cr (p= 0.002) and Sintron (p= 0.03). Yet, Co-Cr and Sintron alloys were not significantly different in this regard (p= 0.22) ( Table 2).
As presented in Figures 5-7, energy dispersive x-ray spectroscopy analysis of the debonded surface of specimens revealed that Silicon (Si) was the most frequently detected element on the debonded surface.

Discussion
The null hypothesis was partially rejected since metalceramic bond strength in non-presintered Co-Cr alloy was significantly higher than that in Co-Cr and Ni-Cr.   Lee et al. [10] investigated the metal-porcelain shear bond strength, and found the shear bond strength of Sintron to be comparable or greater than the casting alloy (4 all). They reported mixed failure mode, which was not analyzed for surface elements based on surface images. In the present study, besides the interpretation of surface images, the cross-sectional image and the debonded surface characteristics were also analyzed, which revealed Silicon (Si) as the most frequently present element on the surface, confirmed the cohesive failure in all three groups. Silicon is the chief element in dental porcelain, which is not generally found in dental alloys, unless contaminated with investments or polishing abrasives used for surface preparation [11].
Marques de Melo et al. [12] investigated the bond strength of four base metal alloys (two Ni-Cr alloys and two Co-Cr alloys) to ceramics, and found no significant difference among them. Similarly, Ahmadzadeh and Ghanavati [13] reported that although Sintron bond strength was lower than Wirobond alloy, it was within the clinically acceptable range. Such a minor difference with the present study might be due to the different design of tests and specimens of the two studies.
In De Melo's study [12], all the four groups (Wiron-99, 4all, Argeloy NP, and IPs d) had cohesive fracture within the porcelain, which is the most favorable type of failure according to Obrien's study [14]. As the well-known base metal alloys, Co-Cr and Ni-Cr have distinct reactions with ceramics [8]. The elements displaced at the metal-ceramic interface accounts for the chemical bond. In the present study, Supremcast V was used as the Ni-Cr alloy, whose bond to porcelain was insignificantly stronger than that of the Wirobond Co-Cr alloy. The presence of nickel, chromium, molybdenum, beryllium and aluminum helps creating a strong chemical bond between this alloy and the ceramic.
Nonetheless, due to biocompatibility and allergy issues of beryllium and nickel, this alloy is better to be cautiously used in high-risk cases.
Fabrication of restorations with sinterable alloy and CAD/CAM includes fewer stages compared with casting alloy restorations, which reduces the probability of error. These restorations are also superior due to the uniformity of their composition at all stages, thanks to the different manufacturing method. In casting, the elements of molten alloy may not be uniformly cooled and change the physical properties of alloy. Furthermore, reactions are likely to occur between the molten metal and the investment material in the casting method. Contamination with wax or resin residues is also possible in the casting method, which affect the mechanical and biological properties of the alloy. All the abovementioned problems can be avoided by using a nonpresintered alloy.
Accordingly, it can be concluded that the sintering process along with CAD/CAM milling used for Sintron has many advantages over the conventional casting method. Sintron is also superior to casting Co-Cr becau-se of its less hardness, which facilitates polishing. Generally, mechanical properties of Sintron are comparable, and in some cases, superior to casting alloys [15].
Among the limitations of the present study was the size of specimens for design and milling, which should be designed by specific software. Further studies are suggested with different alloy and porcelain brands and assessing the failure type in a larger sample size by using energy dispersive x-ray analysis.

Conclusion
With respect to the results of the present study, it can be concluded that besides its superiority in restoration fabrication stages, Sintron bond strength to porcelain is not only comparable to that of casting method, but also higher in vitro. Undoubtedly, dentistry is advancing towards digital approaches so as to minimize the manu-